1. Field of the Invention
The present invention relates to a method for forming hermetic coatings for optical fibers. More specifically, the present invention relates to a method for forming hermetic coatings on optical fibers by filament assisted chemical vapor deposition.
2. Description of Related Art
The reliable performance of silica-based optical fibers is sensitive to factors that effect the long term mechanical strength and time dependent degradation of the optical transmission. It has been shown that exposure to water can reduce the strength of glass fibers with time under stress. It is also known that exposure of the optical fibers to hydrogen gas can degrade the optical transmission of the optical cables. Because of these effects, it is extremely important to isolate the fibers from the surrounding environment by applying a hermetic coating to the glass fiber. To date, coatings made from boron nitride (BN), carbon (C), silicon carbide (SiC) and many other materials have been applied to optical fibers. Although many of these coating have been shown to provide adequate hermetic barriers, in every case they result in a substantial decrease in the measured strength as compared with pristine glass fibers. Kurkjian et al, "Strength and fatigue of silica optical fibers" J. of Lightwave Tech., vol. 7, no. 9, Sep. 1989, pp. 1360-1370. Pristine glass fibers give measure strengths equal to about 10 GPa while the hermetic coated fibers yield strengths in the range of 2-4 GPa.
Other methods for the deposition of other substances have also been studied, such as for example with regard to the production of a coating including carbon (Jansen, "The deposition of diamond films by filament techniques," J. Vac. Sci. Technol. A, vol. 8, no. 5, Sep/Oct 1990 pp. 3785-3790), silicon (Matsumura, "Study on catalytic chemical vapor deposition method to prepare hydrogenated amorphous silicon", J. Appl. Phys., 65(11), 1 Jun. 1989, pp. 4396-4402), and silicon nitride (Matsumura, "Low temperature deposition of silicon nitride by the catalytic chemical vapor deposition method," Jap. J. Appl. Phys., vol. 28, no. 10, Oct., 1989, pp. 2157-2161). U.S. patent application Ser. No. 07/592,457 discloses a method for forming boron nitride coatings using a hot filament, but does not disclose application to optical fibers.
In view of the above, it is desirable to provide a method for forming hermetic coatings for optical fibers which overcome the above-noted deficiencies recognized in conventional techniques with regard to increased fiber strength as compared to pristine glass fibers. It is further desirable to obtain a method which is simple and economical as compared to conventional techniques.